Since nickels are 75% copper 25% nickel even though they have a phase angle similar to gold their response is stronger and more like a penny.

Salt and thin gold has a finite difference and most machines will just not sense the thin gold in the salt but those machines can pick up on thin pieces of cans and foil for similar reasons as nickels.

A good test for a machine in the salt is how well it can detect gold.

SMF's have the advantage of better ID's than SF machines and ib machines have discrimination based on phase angle so we can choose what to dig and what not to dig.

PI machines work purely on conductivity and if you were to discriminate out a nail or deck screw at the beach you would lose gold and probably nickels and small silvers before dropping the nails out because the only discrimination you have is pulse delay. Last bit on Pi machines is you really need to keep the coil close to the ground to get optimal depth with them. Having the coil an inch off the ground or more you will have really poor performance. I literally skim the sand with mine.
Pi machine also doesn't perform as well on dry sand as an ib machine does.